Soldering method is widely used as the means of bonding when electronic components are mounted to a substrate. It is known as one such means of conventional soldering that metal bumps are formed with solder on electronic components to serve as bonding electrodes. Because of miniaturization in size of the bumps for solder bonding due to the advancement of downsizing and compact mounting of electronic components in recent years, it is not avoidable for the bumps to vary in size even among those formed on one and the same electronic component. It is therefore very likely that, if any of the bumps is smaller in size than the other bumps, a gap is left between that bump and a circuit electrode on a substrate when the electronic component is placed on the substrate.
When the bumps are heated for soldering with the gap left open in this manner, the melted solder bumps in a liquid state may cool down and solidify before all the bumps come into contact to surfaces of the circuit electrodes, thereby causing incomplete bonding of the solder. There is a solder bonding method known to prevent such soldering defect, in which the solder bonding portion is supplied with a metallic paste containing metal powder such as silver whose melting point is higher than that of the solder which composes the bumps, when making solder bonding with the bumps. Such method of the solder bonding is disclosed in Japanese Patent Unexamined Publication, No. 2000-114301, for example.
According to this method, melted solder of the bumps flows and spreads over surfaces of the metal powder that remains solid at the moment the bumps melt in the heating process, so that the melted solder is brought to the surface of the circuit electrode even when there is a gap as discussed above, thereby providing an advantage of preventing the soldering defect attributable to the gap between the bump and the circuit electrode.
In the conventional soldering method discussed above, however, the metallic paste needs to contain an active agent to remove oxide films on the surfaces of the solder bumps and to ensure wettability of the bumps in order to allow the melted solder of the bumps to flow through metal powder. There is a case however, that the following problem occurs when flux of a strong activating effect is used.
In recent years, no-cleaning method has become the mainstream in the light of environmental protection and simplification of the working process, which method omits a cleaning process for removing and cleansing flux used for soldering, or the work that has been done previously with a cleansing agent after the solder bonding process. In this no-cleaning method, the flux supplied during the soldering remains intact on the solder bonding portions. If the flux left in this process is very active, it tends to promote deterioration of the insulating property attributable to corrosion of the circuit electrodes on the substrate caused by the residual flux. As described, the conventional soldering method has the problem of causing such troubles as soldering defect and deterioration of the insulating property.